In European Patent Application No. 7930019.0 a laminar support is described and illustrated to provide a credit card or the like having as code and value elements zones or areolae formed by depositing thereon a thin film consisting of ferromagnetic materials or alloys with properties of uniaxial, magnetic anisotropy, i.e. having a rectangular hysteresis loop according to a certain axis parallel to the plane of the thin film and referred to as the easy axis, and a flat or almost null hysteresis loop according to an axis orthogonal to the first one and referred to as the hard axis. This is achieved in a particular environment, e.g. by means of a deposition of a ferromagnetic material or Fe-Ni alloys on a laminar support such as a credit card, an identity card or the like with techniques of electro-deposition or evaporation under hard vacuum and in the presence of a magnetic field tangential to the plane of the support.
The direction of this field will specify the above mentioned easy axis, and the magnetization moment will have only two steady states of orientation according to the two directions of said axis. The film may be deposited on a plurality of zones or areolae, each of them comprising one information unit or bit as a function of both the thickness, and consequently the intensity of the magnetic induction, and the direction of orientation of the easy magnetization axis. In particular, in the specific case of a credit card some areolae can be designed for containing the information relevant to the identification of said credit card, while other areolae are credit or value elements and are designed for progressive cancellation by means of destruction during the services for which the credit card has been provided.
The present invention seeks to provide a device for indentifying some selected characteristics of the material having uniaxial, magnetic anisotropy and deposited on the areolae or zones of a credit card or the like, and eventually for cancelling at least a portion of said areolae.
In this invention a thin film of the above mentioned Fe-Ni alloys having uniaxial, magnetic anisotropy exhibits physical properties such that, if it is subjected to a field produced by a conductor passing a current of frequency f.sub.o and lying on a plane parallel to that of the thin film and oriented according to the easy magnetization axis, it induces in a second conductor lying on the same plane and orthogonal to the first conductor a signal of frequency 2f.sub.o.
This invention utilizes this physical property to provide a non-volatile reading/writing memory, in which the information unit or bit is given by the direction of orientation of the magnetic moment vector of the areola concerned. In such an application the excitation and detection conductors forming the reading/writing device are lying on the same support as the thin film and are integral therewith, their distance from the plane of the thin film being only some microns.
The end result is that of determining only the direction of the magnetic moment vector regardless of an evaluation of the physical quantity concerned.